Conventionally, pilots use weather radars to detect and then avoid hazardous weather. Conventional radar systems may produce the desired results only in a limited environment. Typically, airborne threshold systems are traceable to ground-based weather radar thresholds for wet precipitation generated from convective weather. Aircraft flying substantially above ground and at altitudes far above the normal altitudes for wet precipitation, find that the ground referenced thresholds detect too little weather and the weather that is detected is presented at too low of athreat level. In regions of weather where icy precipitation is melting, these systems may over detect and over warn of weather threats. To better detect and warn during cruise altitude situations many airborne radar systems modified their radar thresholds to produce more detections and greater threat levels at altitude. While making radar performance more acceptable at altitudes, this threshold change over warns and over detects at low altitudes, especially in weather situations where frozen precipitation is melting.
Conventionally, the problem of low sensitivity at cruise altitudes while tending to be overly sensitive at low altitudes has lead to three solutions currently in service today:
1. Set thresholds for one environment only and take what you get in the other environments. This is the xe2x80x9cone size fits allxe2x80x9d solution. Thresholds are typically set as a compromise between high altitude and low altitude performance.
2. Provide a manual gain/threshold control to allow the operator to set the weather representation to match their expectations. Very good performance can be had with this solution but operator training and workload are a problem.
3. Automatically increase sensitivity as aircraft altitude increases. This system can be made to work better than the single threshold solution but still fails to produce robust radar detection in various environments with different weather situations. Typically these systems reduce their sensitivity at long ranges as a function of antenna tilt to keep from over-warning as the radar beam impinges on possible low altitude wet precipitation and on the ground.
Although aircrews desire to use the weather radar as a hazard detector, the radar does not make direct estimates of hazard. Instead, the radar remotely estimates the reflectvity of precipitation in a sampled volume of the atmosphere. A simple model of reflectivity verses precipitation rate can then be used to estimate the rate of precipitation in that atmospheric volume. Historically, high precipitation rates and high radar reflectivity estimates have been associated with two different hazards produced by convective weather; hail and turbulence. Radar systems are calibrated to produce a green display when any precipitation is detected, a yellow display where reflectivity is high enough to produce some chance of a hazard, and a red display when weather produces reflectivity estimates high enough to infer a weather hazard is very likely. The likelihood of weather hazards is based on statistics generated over the North American continent in the spring and summer from radar estimates reflectivities taken from ground based radars. Airborne weathers radars that are used at other altitudes and geographies from where the hazard model data was captured, will generally not capture the threat statistics desired by the aircrew.
Accordingly, there is a need for a system and method to adapt radar thresholds to work in a wider range of environments. Further, there is a need for a system and method that automatically adapts radar thresholds as a function of weather sample altitude, temperature, geography, time of year and it would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claim, regardless of whether they accomplish one or more of the above-mentioned needs.
An example of the invention relates to a method for adapting weather radar gain. The method includes estimating a freezing altitude. The method also includes determining, based on the freezing altitude estimate, the altitude of more than one atmospheric layer. The method further includes determining the proportion of a radar beam sample in each atmospheric layer and adjusting the radar gain, based on the proportion.
Another example of the invention relates to an airborne weather radar system. The system includes a radar antenna on board an aircraft and an adjustable gain circuit for the electromagnetic signal sent via the radar antenna. The adjustable gain circuit is coupled to the radar antenna. The adjustable gain is adjusted based on weather sample altitude and temperature.
Yet another example of the invention relates to a method of adapting radar gains for a weather radar. The method includes measuring a temperature, measuring an altitude, and computing a radar gain based on the temperature and altitude.
Alternative exemplary embodiments relate to other features and combination of features as may be generally recited in the claims.